The present invention relates to methods of RF bias sputtering and, more particularly, to methods of resputtering layers of electrically insulative material used in thin film fabrication operations. More specifically, it relates to the construction of thin film magnetic heads or integrated semiconductor circuits, wherein a passivating or insulating film or layer is RF bias sputter deposited over a polycrystalline or micro-roughened substrate.
The phenomenon of RF bias sputtering or resputtering, in general, is known in the art and involves the reemission of deposited insulative material during the sputter deposition thereof through the effects of attendant ion bombardment of the deposited insulative layer. Apparatus for providing this resputtering characteristic is believed to have been first disclosed in the publication, "Thin Films Deposited by Bias Sputtering," L. I. Maissel, et al, JOURNAL OF APPLIED PHYSICS, January, 1965, p. 237, as a modified DC sputtering technique known as "biased sputtering." The application of the principles of resputtering to RF sputtering is disclosed in an article, "Re-emission Coefficients of Silicon and Silicon Dioxide Films Deposited Through RF and DC Sputtering," R. E. Jones, et al, JOURNAL OF APPLIED PHYSICS, November, 1967, p. 4656, and "Re-emission of Sputtered SiO.sub.2 During Growth and Its Relation to Film Quality," L. I. Maissel, et al, IBM JOURNAL OF RESEARCH AND DEVELOPMENT, March, 1970, p. 176. In effect, resputtering is the positive ion bombardment of a substrate or deposited film during its deposition. Therefore, during RF bias sputtering, there is always simultaneous etching and deposition. The prior art has recognized that RF bias sputtering improves film quality and uniformity, as set forth, for example, in U.S. Pat. No. 3,661,761. The use of sputtering to partially planarize electrical insulative films grossly distorted by underlying integrated circuit lines is taught in U.S. Pat. No. 3,804,738. The use of resputtering to totally remove all elevations from a deposited insulative layer to completely planarize the surface of the layer is taught in copending patent application Ser. No. 512,781, now U.S. Pat. No. 3,983,022, assigned to the same assignee as the present application. In this copending application, silicon dioxide is sputtered onto a metal stripe employing a relatively low amount of resputtering initially and, subsequently, the resputtering is substantially increased. The specific quantitative relationships are not given.
While resputtering has been used to some extent in the fabrication of thin film magnetic heads and integrated circuits, either solely as a deposition technique, for the purpose of improving the quality of sputter deposited films, or for complete or partial planarization of grossly distorted films due to underlying structures, resputtering is not known to have been utilized on polycrystalline or microroughened substrates in a manner which provides a thin insulative film which substantially avoids surface irregularities due to the preferential etching of crystal boundaries or defects on the substrate.
As already noted during RF bias sputtering, there is always simultaneous deposition and etching at the substrate. It has been discovered that polycrystalline ceramic substrates tend to preferentially etch and enlarge their grain boundaries during RF bias sputtering of insulative materials. This causes the substrate to be roughened by deepening and widening the grain boundaries. Subsequently, as the insulative film builds up on the now preferentially etched substrate, the film surface carries substantially the same contours as the preferentially etched substrate and thus may be less smooth than the initial substrate prior to the initiation of RF bias sputtering. The problem of preferential etching is not found to be avoided by polishing of polycrystalline material prior to RF bias sputtering as the micro-smoothness of the substrate does not protect the grain boundaries of polycrystalline ceramic substrate material from being preferentially etched and transmitting itself through the insulative coating. Similarly, micro-roughness or scratches, on the order of 500 A and less on any substrate, whether amorphous, single crystal or polycrystalline, tends to provide a location for preferential etching during RF bias sputtering. Such preferential etching due to micro-roughness also transmits itself upon the surface of the deposited coating. As used herein, the term micro-roughness includes any surface irregularity or scratch in the range of 100-500 A.
Insulative coating may have an effect on subsequently deposited material, and especially on the magnetic properties of subsequently deposited thin magnetic films. It is, therefore, desirable that the grain boundaries or defects in the substrate not transmit themselves on the surface of the deposited insulative material, or even worse, that the grain boundaries and defects not be enlarged so as to increase their effect on the insulative coating due to the natural tendency during RF bias sputtering to preferentially etch along grain boundaries and surface defects.